The present invention relates generally to a system and method for removal of arsenic and fluoride from aqueous solutions, such as drinking water or wastewaters. More specifically, the present invention provides an enhanced system and method of simultaneously removing arsenic and fluoride from aqueous solutions using pretreatment with certain salts and maintaining the solution at certain pH ranges to assist removal of the arsenic and fluoride.
Arsenic bearing aqueous solutions, such as wastewaters, are obtained from a variety of industries including agriculture, mining, semiconductor, and petroleum. Other sources of arsenic bearing surface and groundwaters include natural erosion processes and water obtained from wells. Recent studies on the carcinogenic properties of arsenic (As) have raised concern about the concentration of As in wastewater and drinking water in the US and worldwide. It has been recognized that many potable water sources are contaminated with unacceptable levels of arsenic and may represent a serious health risk.
Arsenic in drinking water is designated as a priority contaminant in the United States under the 1986 Safe Drinking Water Act and amendments thereto. Since 1974, the maximum contaminant level (MCL) for arsenic imposed by the United States Environmental Protection Agency (EPA) is 50 parts per billion (ppb) or (xcexcg/L). As a result of more recent findings pertaining to health risks associated with populations exposed to high concentrations of arsenic in drinking water, the EPA recommends lowering the MCL are arsenic from 50 ppb to 2 ppb. It is expected that in the United States alone, more than 12,000 public water utilities would fail to meet the more stringent proposed arsenic standard. One estimate places the cost of compliance with the proposed MCL standard of 2 ppb to be in excess of $5 billion dollars a year.
The number of private wells in the United States fail to meet the existing MCL for arsenic of 50 ppb, or the proposed MCL of 2 ppb, is unknown. It is believed that in many areas of the United States, many thousand of private wells produce drinking water that is contaminated with arsenic, thus causing potential, serious health risks for the households that depend on this self-produced water.
Further, regionally high arsenic contamination in drinking water is a global problem. In West Bengal, India, for example, an estimated 200,000 people currently suffer from arsenic-induced skin lesions, some of which have advanced to pre-cancerous hyperkeratoses. Accordingly, systems and methods that remove arsenic from aqueous solutions such as drinking water and/or wastewaters are of high importance.
Another significant problem is the presence of fluoride in aqueous solutions, such as drinking water and/or wastewaters. Exposure to fluoride at a concentration of above 1 ppm is toxic to humans. Additionally, fluoride-bearing waters can have a deleterious effect on plants and wildlife.
Industrial processes, especially those used in the manufacture of semiconductors, are a source of wastewater containing both fluoride and arsenic. In particular, devices made from gallium arsenide often used in the telecommunications industry are a significant source of soluble arsenic in wastewater. Ion implantation of arsenic atoms in semiconductor substrates is another source. Fluoride containing chemicals are often used to etch substrates, and to clean materials and the associated processing equipment, both of which lead to fluoride in the wastewaters.
The arsenic atom occurs in four valence states (also called oxidation states); namely, xe2x88x923, 0, +3 and +5. Under standard conditions, the +3 and +5 valence states are respectively found as AsO3xe2x88x923 (arsenite) and AsO4xe2x88x923 (arsenate) ions. For effective arsenic removal by coagulation processes, arsenic should be in the +5 oxidation state, preferably in the form of arsenate. Arsenite is partially removed by techniques such as absorption and coagulation, but the mechanism is less effective because its main form, arsenious acid (H3AsO3), is a weak acid having a pKa1 of about 9.23), and remains unionized at pH values where removal via absorption occurs most effectively; i.e., in the range of about 5 to 8. In contrast, o-arsenic acid (H3AsO4, arsenic in the 5+ oxidation state), is a strong acid (having a pKa1 of about 2.20), and is in an ionized form starting from a pH of approximately 2. The negatively charged form is most effectively absorbed and coagulated.
Various prior art techniques have been employed to remove arsenic from wastewaters. For example, techniques such as co-precipitation, alumina adsorption, and classical ion exchange with anion resins have been used. Such techniques have achieved limited success and are limited to a removal efficiency of only about 95%. Newer techniques have been developed, for example, U.S. Pat. No. 5,368,703 discloses the use of an electrochemical cell which electrochemically generates ferrous ions. A mild oxidizing condition is created by the addition of peroxide which oxidizes the ferrous ions to ferric so that ferric hydroxide is formed. Ferric hydroxide is then used to remove the arsenic. Another prior art technique is described in U.S. Pat. No. 5,908,557 where trivalent arsenic is oxidized to pentavalent arsenic and then removed by a N-alkyl pyridinium containing adsorption medium. Such newer techniques may provide an improvement in the removal efficiency, but such techniques are cumbersome, require specialized equipment and/or specialty chemicals, and are not easily installed or operated, particularly for private well treatment.
Techniques for removing fluoride include precipitating fluoride using calcium and other compounds in single or multiple stage processes, such as that described in U.S. Pat. Nos. 4,145,282, 5,043,072 and 5,403,495. Such techniques vary in the removal efficiency, and do not address the removal of fluoride together with the removal of arsenic.
Accordingly, it would be highly desirable to provide a system and method which is capable of removing both fluoride and arsenic simultaneously from aqueous solutions, in particular from drinking water and industrial wastewaters, to safe levels. It would be further desirable for such a system and method to be flexible and sufficiently robust in order to address the diverse requirements of industry, large municipal water utilities, private wells, and waters in both developed and undeveloped countries.
Accordingly, it is an object of the present invention to provide an improved system and method for removing arsenic and fluoride simultaneously from aqueous solutions, in particular drinking water and wastewaters. More specifically, the inventor has discovered a new system and method of simultaneously removing arsenic and fluoride from aqueous solutions using pretreatment with certain salts and maintaining the aqueous solution at certain pH ranges.
In general, the present invention provides a method and system of removing arsenic and fluoride from an aqueous solution including arsenic in the +5 oxidation state, (the +3 oxidation state may also be present) and fluoride (free fluoride anion) characterized in that the aqueous solution is treated with a combination of calcium salts and then ferric or aluminum salts at a pH in the range of 5 to 8, to form arsenic and fluoride bearing precipitates or solids. The solids are then filtered thereby removing the arsenic and fluoride from the aqueous solution.
In another aspect, the present invention provides a method and system of removing arsenic and fluoride simultaneously from an aqueous solution, comprising the steps of: providing an aqueous solution including arsenic in the form of arsenate ions and fluoride. The pH of the aqueous solution is adjusted to a pH in the range of about 5 to 8. At a pH above 8 the effectiveness of the arsenic removal is significantly reduced. Calcium salts are added to the aqueous solution to promote precipitation of the fluoride to form fluoride bearing particles or floc. Ferric or aluminum based salts are then added to the aqueous solution. The ferric or aluminum based salts form a metal hydroxide floc or suspension which absorbs both the arsenate ions and the fluoride bearing particles to form arsenic and fluoride bearing solids. The arsenic and fluoride bearing solids are then removed from the aqueous solution.
In another aspect of the present invention, a system is provided, comprising a first reaction tank for receiving the aqueous solution including arsenic and fluoride. The pH of the aqueous solution is adjusted to be in a range of 5 to 8, and a first injection device is coupled to the first reaction tank to inject calcium salts into the aqueous solution. A first mixer is preferably coupled to the first reaction tank for mixing the aqueous solution to promote precipitation of the fluoride to form fluoride bearing particles. A second reaction tank is provided for receiving the aqueous solution from the first reaction tank wherein the pH of the aqueous solution is again adjusted as necessary to maintain the pH in the range of about 5 to 8. A second injection device is coupled to the second reaction tank to inject ferric or aluminum based salts into the aqueous solution. Preferably a second mixer is coupled to the second reaction tank to mix the aqueous solution and salts, and a metal hydroxide floc or suspension is formed which absorbs both the arsenic ions arsenic and the fluoride bearing particles to form arsenic and fluoride bearing solids. A filtration system is provided to remove the solids and includes one or more filter vessels having one or more filter membranes arranged in a tubular sock configuration and placed over a slotted tube, and one or more settling tanks.